BATC - Forum

A place for ATV enthusiasts to get together and discuss all things Television. NOTE - all postings remain the copyright of the author, please get their permission before copying and publishing elsewhere.

Hi,
Looking for suggestions for an aerial /dish for 23cm ATV.
Looking at either buying one or building one - don't mind either way.
Ideally something not too expensive as just looking to dabble at receiving 23cm ATV for now.

Chris have a look here http://www.w6ze.org/DATV/TechTalk99-DATV.pdf and also search for the original CQTV article by Paul G8GML.
I've been using two of these antennas one for TX one for RX for between 15 and 20 years. They are still up on the mast and working fine, better than any Yagi I ever tried! I use the 6 loop 4 bay design. It's really easy to make a bit of chicken wire, some square alloy tube, coax centre copper wire for the loops, plastic rawl plugs for standoffs. I bypassed the matching recommendations by finding suitable 4 port splitters at rallys. If you're into a bit of DIY You'll enjoy making one.
You could try just a single bay 5 loop to start with, there are no matching problems with it. Good luck.

CQ-TV 258 page 34 seems to consist of a Sky dish, sanitary fittings and a bowtie aerial. Kevin G7VNP looks to have been ingenious. I'll have a go once the VNA arrives from China. And I've acquired a scrap dish.

I'm making JVL 23cm quad loop antennas at the moment using JVL's program to design them. As I couldn't find much about measured performance of the narrowband (NB) and wideband (WB) designs, I'm going to try both. I'm building them with aluminium strip elements (apart from the DE which is copper).

My first attempt is a 26el narrowband design for 1296.2MHz on a 2.2m long 20mm square boom. As built, its minimum return loss was on 1301MHz; with some flattening of the DE it's spot on 1296 with Rl of 30dB indicated on my Siglent SA. The real number is less than that as I have some feeder loss. Usable bandwidth (arbitrarily taking Rl at least 15dB) is about 6MHz allowing for the feeder flattering. It has directivity and receives a local beacon when 2m off the ground, so I guess it works.

With some extended driving of the JVL program, I'd noticed that the director spacing doesn't change between NB and WB version, though there is a small offset. I've therefore made the antenna with a removable feed section so I can swap between NB and WB.

I'm now looking at the WB feed with the 3el log periodic cell. I've noticed that the program results in uneven spacing of the 3 elements (as in G0NMY's photos), and the front one is only 18mm from D1. Looking at pictures of the JVL-manufactured WB antennas, they're not like this. The spacing is uniform and the distance between the front DE and D1 is bigger than the program tells me. For example G8VPG's photo here shows this viewtopic.php?f=2&t=5117

My question is, has anyone made a WB quad loop using the program and got it to work? I note G0NMY's photos of the feed beautifully made - have you tried it yet? I live in Southampton and Dave G8GKQ has offered me some measuring time on his JVL-built WB antenna; I hope to do this over the coming weeks (Grandchildren holiday duty looming ...).

Hi Chris,
Well I contacted Michael G3JVL as I was confused about the position of the wideband element position to the refelector plate.
The Position given in the program for the wideband driven element is from the reflector plate to the actual feed point itself
Where as I thought it was to the back of the 3 loops. Michael is 80+ years old and admits his memory is not as good as it was but was certain
of the feed point being the correct spacing as specified.

The images I posted are not mine, they are what G3JVL sent to me in an email when I was unsure of the designs spacings and how the feed was made.Infact the pictures are for a 13cms unit according to Mike.

I dont have any antenna analyser for 1.2GHz but checking vswr showed excellent bandwidth I had to borrow a vswr meter from a fellow ham.
The wideband feed is required because of txing on 1249MHz and RXing on 1318MHz GB3UD
They do work very well but my build quality failed so I have to rebuild a new one. I also fancy one for 5.6GHz it should be quite small but mounting the elements accurately may be an issue.
Anyway Good luck with your build and hope it works well for you.

I've built a log periodic feed as directed by the JVL program, and made some measurements on this and a JVL original (thanks to Dave G8GKQ for the use of his). I've looked at the dimensions of the log periodic feed as calculated by the JVL program, as used in Dave's JVL original, and also at the dimensions reported by G8VPG for his JVL-built model. I compared these with the standard equations for log periodic arrays. My findings thus far are:

a) neither my model with strip elements, nor Dave's JVL with wire loops have what I'd consider a "good" match across the band. Return loss plots are below (apologies - the frequency scales aren't the same). 10dB is about 2:1 SWR, 15dB is about 1.5:1, 20dB is about 1.2:1). Having said that, Dave has been using his successfully for years!

b) The whole plot is too lf, and the nulls in the Rl plots look too far apart to me. If they were closer, I'd expect the Rl between to be better. However with a couple of days re-building (and trying 2.5mm wire loops) I haven't managed to make any significant improvement. I can't grasp the what-changes-what and the dimensions all interact, so try-it-and-see isn't likely to get me to the answer. The dimensions don't seem to comply with the standard log periodic design equations.

c) the two JVL-built models have different dimensions which are different again to those calculated by the program. For example, the spacing between the front driven loop and first director is 18mm from the program, and 33mm for the JVL-built models. Inter-loop spacing is 40 & 20 from the program, 30 & 25 for Dave's, and 27 & 27 for G8VPG's. Loop sizes are also different.

I have paused at this point to get on building the rest of the 23cm gear and the narrowband feed is back on the antenna. I hope to be able to do some pattern measurements next weekend if the wx is ok and we can get an antenna range set up. I'd like to return to the log periodic feed later. G3JVL lives not far from me and I will enquire if he'd mind a visit for some wisdom.